Computer_Networking_A_Top-Down_Approach.pdf

COMPUTER NETWORKING SIXTH EDITION A Top-Down Approach James F. Kurose University of Massachusetts, Amherst Keith W. Ross Polytechnic Institute of NYU Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montréal Toronto Delhi Mexico City São Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo Vice President and Editorial Director, ECS: Marcia Horton Editor in Chief: Michael Hirsch Editorial Assistant: Emma Snider Vice President Marketing: Patrice Jones Marketing Manager: Yez Alayan Marketing Coordinator: Kathryn Ferranti Vice President and Director of Production: Vince O’Brien Managing Editor: Jeff Holcomb Senior Production Project Manager: Marilyn Lloyd Manufacturing Manager: Nick Sklitsis Operations Specialist: Lisa McDowell Art Director, Cover: Anthony Gemmellaro Art Coordinator: Janet Theurer/ Theurer Briggs Design Art Studio: Patrice Rossi Calkin/ Rossi Illustration and Design Cover Designer: Liz Harasymcuk Text Designer: Joyce Cosentino Wells Cover Image: ©Fancy/Alamy Media Editor: Dan Sandin Full-Service Vendor: PreMediaGlobal Senior Project Manager: Andrea Stefanowicz Printer/Binder: Edwards Brothers Cover Printer: Lehigh-Phoenix Color This book was composed in Quark. Basal font is Times. Display font is Berkeley. Copyright © 2013, 2010, 2008, 2005, 2003 by Pearson Education, Inc., publishing as Addison-Wesley. All rights reserved. Manufactured in the United States of America. This publication is protected by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtain permission(s) to use material from this work, please submit a written request to Pearson Education, Inc., Permissions Department, One Lake Street, Upper Saddle River, New Jersey 07458, or you may fax your request to 201-236-3290. Many of the designations by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed in initial caps or all caps. Library of Congress Cataloging-in-Publication Data Kurose, James F. Computer networking : a top-down approach / James F. Kurose, Keith W. Ross.—6th ed. p. cm. Includes bibliographical references and index. ISBN-13: 978-0-13-285620-1 ISBN-10: 0-13-285620-4 1. Internet. 2. Computer networks. I. Ross, Keith W., 1956- II. Title. TK5105.875.I57K88 2012 004.6—dc23 2011048215 10 9 8 7 6 5 4 3 2 1 ISBN-10: 0-13-285620-4 ISBN-13: 978-0-13-285620-1 About the Authors Jim Kurose Jim Kurose is a Distinguished University Professor of Computer Science at the University of Massachusetts, Amherst. Dr. Kurose has received a number of recognitions for his educational activities including Outstanding Teacher Awards from the National Technological University (eight times), the University of Massachusetts, and the Northeast Association of Graduate Schools. He received the IEEE Taylor Booth Education Medal and was recognized for his leadership of Massachusetts’ Commonwealth Information Technology Initiative. He has been the recipient of a GE Fellowship, an IBM Faculty Development Award, and a Lilly Teaching Fellowship. Dr. Kurose is a former Editor-in-Chief of IEEE Transactions on Communications and of IEEE/ACM Transactions on Networking. He has been active in the program committees for IEEE Infocom, ACM SIGCOMM, ACM Internet Measurement Conference, and ACM SIGMETRICS for a number of years and has served as Technical Program Co-Chair for those conferences. He is a Fellow of the IEEE and the ACM. His research interests include network protocols and architecture, network measurement, sensor networks, multimedia communication, and modeling and performance evaluation. He holds a PhD in Computer Science from Columbia University. Keith Ross Keith Ross is the Leonard J. Shustek Chair Professor and Head of the Computer Science Department at Polytechnic Institute of NYU. Before joining NYU-Poly in 2003, he was a professor at the University of Pennsylvania (13 years) and a professor at Eurecom Institute (5 years). He received a B.S.E.E from Tufts University, a M.S.E.E. from Columbia University, and a Ph.D. in Computer and Control Engineering from The University of Michigan. Keith Ross is also the founder and original CEO of Wimba, which develops online multimedia applications for e-learning and was acquired by Blackboard in 2010. Professor Ross’s research interests are in security and privacy, social networks, peer-to-peer networking, Internet measurement, video streaming, content distribution networks, and stochastic modeling. He is an IEEE Fellow, recipient of the Infocom 2009 Best Paper Award, and recipient of 2011 and 2008 Best Paper Awards for Multimedia Communications (awarded by IEEE Communications Society). He has served on numerous journal editorial boards and conference program committees, including IEEE/ACM Transactions on Networking, ACM SIGCOMM, ACM CoNext, and ACM Internet Measurement Conference. He also has served as an advisor to the Federal Trade Commission on P2P file sharing. iii This page intentionally left blank To Julie and our three precious ones—Chris, Charlie, and Nina JFK A big THANKS to my professors, colleagues, and students all over the world. KWR This page intentionally left blank Preface Welcome to the sixth edition of Computer Networking: A Top-Down Approach. Since the publication of the first edition 12 years ago, our book has been adopted for use at many hundreds of colleges and universities, translated into 14 languages, and used by over one hundred thousand students and practitioners worldwide. We’ve heard from many of these readers and have been overwhelmed by the positive response. What’s New in the Sixth Edition? We think one important reason for this success has been that our book continues to offer a fresh and timely approach to computer networking instruction. We’ve made changes in this sixth edition, but we’ve also kept unchanged what we believe (and the instructors and students who have used our book have confirmed) to be the most important aspects of this book: its top-down approach, its focus on the Internet and a modern treatment of computer networking, its attention to both principles and practice, and its accessible style and approach toward learning about computer networking. Nevertheless, the sixth edition has been revised and updated substantially: The Companion Web site has been significantly expanded and enriched to include VideoNotes and interactive exercises, as discussed later in this Preface. In Chapter 1, the treatment of access networks has been modernized, and the description of the Internet ISP ecosystem has been substantially revised, accounting for the recent emergence of content provider networks, such as Google’s. The presentation of packet switching and circuit switching has also been reorganized, providing a more topical rather than historical orientation. In Chapter 2, Python has replaced Java for the presentation of socket programming. While still explicitly exposing the key ideas behind the socket API, Python code is easier to understand for the novice programmer. Moreover, unlike Java, Python provides access to raw sockets, enabling students to build a larger variety of network applications. Java-based socket programming labs have been replaced with corresponding Python labs, and a new Python-based ICMP Ping lab has been added. As always, when material is retired from the book, such as Java-based socket programming material, it remains available on the book’s Companion Web site (see following text). In Chapter 3, the presentation of one of the reliable data transfer protocols has been simplified and a new sidebar on TCP splitting, commonly used to optimize the performance of cloud services, has been added. In Chapter 4, the section on router architectures has been significantly updated, reflecting recent developments and practices in the field. Several new integrative sidebars involving DNS, BGP, and OSPF are included. viii Preface Chapter 5 has been reorganized and streamlined, accounting for the ubiquity of switched Ethernet in local area networks and the consequent increased use of Ethernet in point-to-point scenarios. Also, a new section on data center networking has been added. Chapter 6 has been updated to reflect recent advances in wireless networks, particularly cellular data networks and 4G services and architecture. Chapter 7, which focuses on multimedia networking, has gone through a major revision. The chapter now includes an in-depth discussion of streaming video, including adaptive streaming, and an entirely new and modernized discussion of CDNs. A newly added section describes the Netflix, YouTube, and Kankan video streaming systems. The material that has been removed to make way for these new topics is still available on the Companion Web site. Chapter 8 now contains an expanded discussion on endpoint authentication. Significant new material involving end-of-chapter problems has been added. As with all previous editions, homework problems have been revised, added, and removed. Audience This textbook is for a first course on computer networking. It can be used in both computer science and electrical engineering departments. In terms of programming languages, the book assumes only that the student has experience with C, C++, Java, or Python (and even then only in a few places). Although this book is more precise and analytical than many other introductory computer networking texts, it rarely uses any mathematical concepts that are not taught in high school. We have made a deliberate effort to avoid using any advanced calculus, probability, or stochastic process concepts (although we’ve included some homework problems for students with this advanced background). The book is therefore appropriate for undergraduate courses and for first-year graduate courses. It should also be useful to practitioners in the telecommunications industry. What Is Unique about This Textbook? The subject of computer networking is enormously complex, involving many concepts, protocols, and technologies that are woven together in an intricate manner. To cope with this scope and complexity, many computer networking texts are often organized around the “layers” of a network architecture. With a layered organization, students can see through the complexity of computer networking— they learn about the distinct concepts and protocols in one part of the architecture while seeing the big picture of how all parts fit together. From a pedagogical perspective, our personal experience has been that such a layered approach Preface indeed works well. Nevertheless, we have found that the traditional approach of teaching—bottom up; that is, from the physical layer towards the application layer—is not the best approach for a modern course on computer networking. A Top-Down Approach Our book broke new ground 12 years ago by treating networking in a top-down manner—that is, by beginning at the application layer and working its way down toward the physical layer. The feedback we received from teachers and students alike have confirmed that this top-down approach has many advantages and does indeed work well pedagogically. First, it places emphasis on the application layer (a “high growth area” in networking). Indeed, many of the recent revolutions in computer networking—including the Web, peer-to-peer file sharing, and media streaming—have taken place at the application layer. An early emphasis on applicationlayer issues differs from the approaches taken in most other texts, which have only a small amount of material on network applications, their requirements, application-layer paradigms (e.g., client-server and peer-to-peer), and application programming interfaces. Second, our experience as instructors (and that of many instructors who have used this text) has been that teaching networking applications near the beginning of the course is a powerful motivational tool. Students are thrilled to learn about how networking applications work—applications such as e-mail and the Web, which most students use on a daily basis. Once a student understands the applications, the student can then understand the network services needed to support these applications. The student can then, in turn, examine the various ways in which such services might be provided and implemented in the lower layers. Covering applications early thus provides motivation for the remainder of the text. Third, a top-down approach enables instructors to introduce network application development at an early stage. Students not only see how popular applications and protocols work, but also learn how easy it is to create their own network applications and application-level protocols. With the top-down approach, students get early exposure to the notions of socket programming, service models, and protocols—important concepts that resurface in all subsequent layers. By providing socket programming examples in Python, we highlight the central ideas without confusing students with complex code. Undergraduates in electrical engineering and computer science should not have difficulty following the Python code. An Internet Focus Although we dropped the phrase “Featuring the Internet” from the title of this book with the fourth edition, this doesn’t mean that we dropped our focus on the Internet! Indeed, nothing could be further from the case! Instead, since the Internet has become so pervasive, we felt that any networking textbook must have a significant ix x Preface focus on the Internet, and thus this phrase was somewhat unnecessary. We continue to use the Internet’s architecture and protocols as primary vehicles for studying fundamental computer networking concepts. Of course, we also include concepts and protocols from other network architectures. But the spotlight is clearly on the Internet, a fact reflected in our organizing the book around the Internet’s five-layer architecture: the application, transport, network, link, and physical layers. Another benefit of spotlighting the Internet is that most computer science and electrical engineering students are eager to learn about the Internet and its protocols. They know that the Internet has been a revolutionary and disruptive technology and can see that it is profoundly changing our world. Given the enormous relevance of the Internet, students are naturally curious about what is “under the hood.” Thus, it is easy for an instructor to get students excited about basic principles when using the Internet as the guiding focus. Teaching Networking Principles Two of the unique features of the book—its top-down approach and its focus on the Internet—have appeared in the titles of our book. If we could have squeezed a third phrase into the subtitle, it would have contained the word principles. The field of networking is now mature enough that a number of fundamentally important issues can be identified. For example, in the transport layer, the fundamental issues include reliable communication over an unreliable network layer, connection establishment/ teardown and handshaking, congestion and flow control, and multiplexing. Two fundamentally important network-layer issues are determining “good” paths between two routers and interconnecting a large number of heterogeneous networks. In the link layer, a fundamental problem is sharing a multiple access channel. In network security, techniques for providing confidentiality, authentication, and message integrity are all based on cryptographic fundamentals. This text identifies fundamental networking issues and studies approaches towards addressing these issues. The student learning these principles will gain knowledge with a long “shelf life”—long after today’s network standards and protocols have become obsolete, the principles they embody will remain important and relevant. We believe that the combination of using the Internet to get the student’s foot in the door and then emphasizing fundamental issues and solution approaches will allow the student to quickly understand just about any networking technology. The Web Site Each new copy of this textbook includes six months of access to a Companion Web site for all book readers at http://www.pearsonhighered.com/kurose-ross, which includes: Interactive learning material. An important new component of the sixth edition is the significantly expanded online and interactive learning material. The book’s Companion Web site now contains VideoNotes—video presentations of Preface important topics thoughout the book done by the authors, as well as walkthroughs of solutions to problems similar to those at the end of the chapter. We’ve also added Interactive Exercises that can create (and present solutions for) problems similar to selected end-of-chapter problems. Since students can generate (and view solutions for) an unlimited number of similar problem instances, they can work until the material is truly mastered. We’ve seeded the Web site with VideoNotes and online problems for chapters 1 through 5 and will continue to actively add and update this material over time. As in earlier editions, the Web site contains the interactive Java applets that animate many key networking concepts. The site also has interactive quizzes that permit students to check their basic understanding of the subject matter. Professors can integrate these interactive features into their lectures or use them as mini labs. Additional technical material. As we have added new material in each edition of our book, we’ve had to remove coverage of some existing topics to keep the book at manageable length. For example, to make room for the new material in this edition, we’ve removed material on ATM networks and the RTSP protocol for multimedia. Material that appeared in earlier editions of the text is still of interest, and can be found on the book’s Web site. Programming assignments. The Web site also provides a number of detailed programming assignments, which include building a multithreaded Web server, building an e-mail client with a GUI interface, programming the sender and receiver sides of a reliable data transport protocol, programming a distributed routing algorithm, and more. Wireshark labs. One’s understanding of network protocols can be greatly deepened by seeing them in action. The Web site provides numerous Wireshark assignments that enable students to actually observe the sequence of messages exchanged between two protocol entities. The Web site includes separate Wireshark labs on HTTP, DNS, TCP, UDP, IP, ICMP, Ethernet, ARP, WiFi, SSL, and on tracing all protocols involved in satisfying a request to fetch a web page. We’ll continue to add new labs over time. Pedagogical Features We have each been teaching computer networking for more than 20 years. Together, we bring more than 50 years of teaching experience to this text, during which time we have taught many thousands of students. We have also been active researchers in computer networking during this time. (In fact, Jim and Keith first met each other as master’s students in a computer networking course taught by Mischa Schwartz in 1979 at Columbia University.) We think all this gives us a good perspective on where networking has been and where it is likely to go in the future. Nevertheless, we have resisted temptations to bias the material in this book xi xii Preface towards our own pet research projects. We figure you can visit our personal Web sites if you are interested in our research. Thus, this book is about modern computer networking—it is about contemporary protocols and technologies as well as the underlying principles behind these protocols and technologies. We also believe that learning (and teaching!) about networking can be fun. A sense of humor, use of analogies, and real-world examples in this book will hopefully make this material more fun. Supplements for Instructors We provide a complete supplements package to aid instructors in teaching this course. This material can be accessed from Pearson’s Instructor Resource Center (http://www.pearsonhighered.com/irc). Visit the Instructor Resource Center or send e-mail to [email protected] for information about accessing these instructor’s supplements. PowerPoint® slides. We provide PowerPoint slides for all nine chapters. The slides have been completely updated with this sixth edition. The slides cover each chapter in detail. They use graphics and animations (rather than relying only on monotonous text bullets) to make the slides interesting and visually appealing. We provide the original PowerPoint slides so you can customize them to best suit your own teaching needs. Some of these slides have been contributed by other instructors who have taught from our book. Homework solutions. We provide a solutions manual for the homework problems in the text, programming assignments, and Wireshark labs. As noted earlier, we’ve introduced many new homework problems in the first five chapters of the book. Chapter Dependencies The first chapter of this text presents a self-contained overview of computer networking. Introducing many key concepts and terminology, this chapter sets the stage for the rest of the book. All of the other chapters directly depend on this first chapter. After completing Chapter 1, we recommend instructors cover Chapters 2 through 5 in sequence, following our top-down philosophy. Each of these five chapters leverages material from the preceding chapters. After completing the first five chapters, the instructor has quite a bit of flexibility. There are no interdependencies among the last four chapters, so they can be taught in any order. However, each of the last four chapters depends on the material in the first five chapters. Many instructors first teach the first five chapters and then teach one of the last four chapters for “dessert.” Preface One Final Note: We’d Love to Hear from You We encourage students and instructors to e-mail us with any comments they might have about our book. It’s been wonderful for us to hear from so many instructors and students from around the world about our first four editions. We’ve incorporated many of these suggestions into later editions of the book. We also encourage instructors to send us new homework problems (and solutions) that would complement the current homework problems. We’ll post these on the instructor-only portion of the Web site. We also encourage instructors and students to create new Java applets that illustrate the concepts and protocols in this book. If you have an applet that you think would be appropriate for this text, please submit it to us. If the applet (including notation and terminology) is appropriate, we’ll be happy to include it on the text’s Web site, with an appropriate reference to the applet’s authors. So, as the saying goes, “Keep those cards and letters coming!” Seriously, please do continue to send us interesting URLs, point out typos, disagree with any of our claims, and tell us what works and what doesn’t work. Tell us what you think should or shouldn’t be included in the next edition. Send your e-mail to [email protected] and [email protected]. Acknowledgments Since we began writing this book in 1996, many people have given us invaluable help and have been influential in shaping our thoughts on how to best organize and teach a networking course. We want to say A BIG THANKS to everyone who has helped us from the earliest first drafts of this book, up to this fifth edition. We are also very thankful to the many hundreds of readers from around the world—students, faculty, practitioners—who have sent us thoughts and comments on earlier editions of the book and suggestions for future editions of the book. Special thanks go out to: Al Aho (Columbia University) Hisham Al-Mubaid (University of Houston-Clear Lake) Pratima Akkunoor (Arizona State University) Paul Amer (University of Delaware) Shamiul Azom (Arizona State University) Lichun Bao (University of California at Irvine) Paul Barford (University of Wisconsin) Bobby Bhattacharjee (University of Maryland) Steven Bellovin (Columbia University) Pravin Bhagwat (Wibhu) Supratik Bhattacharyya (previously at Sprint) Ernst Biersack (Eurécom Institute) xiii xiv Preface Shahid Bokhari (University of Engineering & Technology, Lahore) Jean Bolot (Technicolor Research) Daniel Brushteyn (former University of Pennsylvania student) Ken Calvert (University of Kentucky) Evandro Cantu (Federal University of Santa Catarina) Jeff Case (SNMP Research International) Jeff Chaltas (Sprint) Vinton Cerf (Google) Byung Kyu Choi (Michigan Technological University) Bram Cohen (BitTorrent, Inc.) Constantine Coutras (Pace University) John Daigle (University of Mississippi) Edmundo A. de Souza e Silva (Federal University of Rio de Janeiro) Philippe Decuetos (Eurécom Institute) Christophe Diot (Technicolor Research) Prithula Dhunghel (Akamai) Deborah Estrin (University of California, Los Angeles) Michalis Faloutsos (University of California at Riverside) Wu-chi Feng (Oregon Graduate Institute) Sally Floyd (ICIR, University of California at Berkeley) Paul Francis (Max Planck Institute) Lixin Gao (University of Massachusetts) JJ Garcia-Luna-Aceves (University of California at Santa Cruz) Mario Gerla (University of California at Los Angeles) David Goodman (NYU-Poly) Yang Guo (Alcatel/Lucent Bell Labs) Tim Griffin (Cambridge University) Max Hailperin (Gustavus Adolphus College) Bruce Harvey (Florida A&M University, Florida State University) Carl Hauser (Washington State University) Rachelle Heller (George Washington University) Phillipp Hoschka (INRIA/W3C) Wen Hsin (Park University) Albert Huang (former University of Pennsylvania student) Cheng Huang (Microsoft Research) Esther A. Hughes (Virginia Commonwealth University) Van Jacobson (Xerox PARC) Pinak Jain (former NYU-Poly student) Jobin James (University of California at Riverside) Sugih Jamin (University of Michigan) Shivkumar Kalyanaraman (IBM Research, India) Jussi Kangasharju (University of Helsinki) Sneha Kasera (University of Utah) Parviz Kermani (formerly of IBM Research) Preface Hyojin Kim (former University of Pennsylvania student) Leonard Kleinrock (University of California at Los Angeles) David Kotz (Dartmouth College) Beshan Kulapala (Arizona State University) Rakesh Kumar (Bloomberg) Miguel A. Labrador (University of South Florida) Simon Lam (University of Texas) Steve Lai (Ohio State University) Tom LaPorta (Penn State University) Tim-Berners Lee (World Wide Web Consortium) Arnaud Legout (INRIA) Lee Leitner (Drexel University) Brian Levine (University of Massachusetts) Chunchun Li (former NYU-Poly student) Yong Liu (NYU-Poly) William Liang (former University of Pennsylvania student) Willis Marti (Texas A&M University) Nick McKeown (Stanford University) Josh McKinzie (Park University) Deep Medhi (University of Missouri, Kansas City) Bob Metcalfe (International Data Group) Sue Moon (KAIST) Jenni Moyer (Comcast) Erich Nahum (IBM Research) Christos Papadopoulos (Colorado Sate University) Craig Partridge (BBN Technologies) Radia Perlman (Intel) Jitendra Padhye (Microsoft Research) Vern Paxson (University of California at Berkeley) Kevin Phillips (Sprint) George Polyzos (Athens University of Economics and Business) Sriram Rajagopalan (Arizona State University) Ramachandran Ramjee (Microsoft Research) Ken Reek (Rochester Institute of Technology) Martin Reisslein (Arizona State University) Jennifer Rexford (Princeton University) Leon Reznik (Rochester Institute of Technology) Pablo Rodrigez (Telefonica) Sumit Roy (University of Washington) Avi Rubin (Johns Hopkins University) Dan Rubenstein (Columbia University) Douglas Salane (John Jay College) Despina Saparilla (Cisco Systems) John Schanz (Comcast) xv xvi Preface Henning Schulzrinne (Columbia University) Mischa Schwartz (Columbia University) Ardash Sethi (University of Delaware) Harish Sethu (Drexel University) K. Sam Shanmugan (University of Kansas) Prashant Shenoy (University of Massachusetts) Clay Shields (Georgetown University) Subin Shrestra (University of Pennsylvania) Bojie Shu (former NYU-Poly student) Mihail L. Sichitiu (NC State University) Peter Steenkiste (Carnegie Mellon University) Tatsuya Suda (University of California at Irvine) Kin Sun Tam (State University of New York at Albany) Don Towsley (University of Massachusetts) David Turner (California State University, San Bernardino) Nitin Vaidya (University of Illinois) Michele Weigle (Clemson University) David Wetherall (University of Washington) Ira Winston (University of Pennsylvania) Di Wu (Sun Yat-sen University) Shirley Wynn (NYU-Poly) Raj Yavatkar (Intel) Yechiam Yemini (Columbia University) Ming Yu (State University of New York at Binghamton) Ellen Zegura (Georgia Institute of Technology) Honggang Zhang (Suffolk University) Hui Zhang (Carnegie Mellon University) Lixia Zhang (University of California at Los Angeles) Meng Zhang (former NYU-Poly student) Shuchun Zhang (former University of Pennsylvania student) Xiaodong Zhang (Ohio State University) ZhiLi Zhang (University of Minnesota) Phil Zimmermann (independent consultant) Cliff C. Zou (University of Central Florida) We also want to thank the entire Addison-Wesley team—in particular, Michael Hirsch, Marilyn Lloyd, and Emma Snider—who have done an absolutely outstanding job on this sixth edition (and who have put up with two very finicky authors who seem congenitally unable to meet deadlines!). Thanks also to our artists, Janet Theurer and Patrice Rossi Calkin, for their work on the beautiful figures in this book, and to Andrea Stefanowicz and her team at PreMediaGlobal for their wonderful production work on this edition. Finally, a most special thanks go to Michael Hirsch, our editor at AddisonWesley, and Susan Hartman, our former editor at Addison-Wesley. This book would not be what it is (and may well not have been at all) without their graceful management, constant encouragement, nearly infinite patience, good humor, and perseverance. Table of Contents Chapter 1 Computer Networks and the Internet 1.1 What Is the Internet? 1.1.1 A Nuts-and-Bolts Description 1.1.2 A Services Description 1.1.3 What Is a Protocol? 1.2 The Network Edge 1.2.1 Access Networks 1.2.2 Physical Media 1.3 The Network Core 1.3.1 Packet Switching 1.3.2 Circuit Switching 1.3.3 A Network of Networks 1.4 Delay, Loss, and Throughput in Packet-Switched Networks 1.4.1 Overview of Delay in Packet-Switched Networks 1.4.2 Queuing Delay and Packet Loss 1.4.3 End-to-End Delay 1.4.4 Throughput in Computer Networks 1.5 Protocol Layers and Their Service Models 1.5.1 Layered Architecture 1.5.2 Encapsulation 1.6 Networks Under Attack 1.7 History of Computer Networking and the Internet 1.7.1 The Development of Packet Switching: 1961–1972 1.7.2 Proprietary Networks and Internetworking: 1972–1980 1.7.3 A Proliferation of Networks: 1980–1990 1.7.4 The Internet Explosion: The 1990s 1.7.5 The New Millennium 1.8 Summary Homework Problems and Questions Wireshark Lab Interview: Leonard Kleinrock 1 2 2 5 7 9 12 18 22 22 27 32 35 35 39 42 44 47 47 53 55 60 60 62 63 64 65 66 68 78 80 xvii xviii Table of Contents Chapter 2 Application Layer 2.1 Principles of Network Applications 2.1.1 Network Application Architectures 2.1.2 Processes Communicating 2.1.3 Transport Services Available to Applications 2.1.4 Transport Services Provided by the Internet 2.1.5 Application-Layer Protocols 2.1.6 Network Applications Covered in This Book 2.2 The Web and HTTP 2.2.1 Overview of HTTP 2.2.2 Non-Persistent and Persistent Connections 2.2.3 HTTP Message Format 2.2.4 User-Server Interaction: Cookies 2.2.5 Web Caching 2.2.6 The Conditional GET 2.3 File Transfer: FTP 2.3.1 FTP Commands and Replies 2.4 Electronic Mail in the Internet 2.4.1 SMTP 2.4.2 Comparison with HTTP 2.4.3 Mail Message Format 2.4.4 Mail Access Protocols 2.5 DNS—The Internet’s Directory Service 2.5.1 Services Provided by DNS 2.5.2 Overview of How DNS Works 2.5.3 DNS Records and Messages 2.6 Peer-to-Peer Applications 2.6.1 P2P File Distribution 2.6.2 Distributed Hash Tables (DHTs) 2.7 Socket Programming: Creating Network Applications 2.7.1 Socket Programming with UDP 2.7.2 Socket Programming with TCP 2.8 Summary Homework Problems and Questions Socket Programming Assignments Wireshark Labs: HTTP, DNS Interview: Marc Andreessen 83 84 86 88 91 93 96 97 98 98 100 103 108 110 114 116 118 118 121 124 125 125 130 131 133 139 144 145 151 156 157 163 168 169 179 181 182 Table of Contents Chapter 3 Transport Layer 3.1 Introduction and Transport-Layer Services 3.1.1 Relationship Between Transport and Network Layers 3.1.2 Overview of the Transport Layer in the Internet 3.2 Multiplexing and Demultiplexing 3.3 Connectionless Transport: UDP 3.3.1 UDP Segment Structure 3.3.2 UDP Checksum 3.4 Principles of Reliable Data Transfer 3.4.1 Building a Reliable Data Transfer Protocol 3.4.2 Pipelined Reliable Data Transfer Protocols 3.4.3 Go-Back-N (GBN) 3.4.4 Selective Repeat (SR) 3.5 Connection-Oriented Transport: TCP 3.5.1 The TCP Connection 3.5.2 TCP Segment Structure 3.5.3 Round-Trip Time Estimation and Timeout 3.5.4 Reliable Data Transfer 3.5.5 Flow Control 3.5.6 TCP Connection Management 3.6 Principles of Congestion Control 3.6.1 The Causes and the Costs of Congestion 3.6.2 Approaches to Congestion Control 3.6.3 Network-Assisted Congestion-Control Example: ATM ABR Congestion Control 3.7 TCP Congestion Control 3.7.1 Fairness 3.8 Summary Homework Problems and Questions Programming Assignments Wireshark Labs: TCP, UDP Interview: Van Jacobson Chapter 4 The Network Layer 4.1 4.2 Introduction 4.1.1 Forwarding and Routing 4.1.2 Network Service Models Virtual Circuit and Datagram Networks 4.2.1 Virtual-Circuit Networks 4.2.2 Datagram Networks 4.2.3 Origins of VC and Datagram Networks xix 185 186 186 189 191 198 202 202 204 206 215 218 223 230 231 233 238 242 250 252 259 259 265 266 269 279 283 285 300 301 302 305 306 308 310 313 314 317 319 xx Table of Contents 4.3 What’s Inside a Router? 4.3.1 Input Processing 4.3.2 Switching 4.3.3 Output Processing 4.3.4 Where Does Queuing Occur? 4.3.5 The Routing Control Plane 4.4 The Internet Protocol (IP): Forwarding and Addressing in the Internet 4.4.1 Datagram Format 4.4.2 IPv4 Addressing 4.4.3 Internet Control Message Protocol (ICMP) 4.4.4 IPv6 4.4.5 A Brief Foray into IP Security 4.5 Routing Algorithms 4.5.1 The Link-State (LS) Routing Algorithm 4.5.2 The Distance-Vector (DV) Routing Algorithm 4.5.3 Hierarchical Routing 4.6 Routing in the Internet 4.6.1 Intra-AS Routing in the Internet: RIP 4.6.2 Intra-AS Routing in the Internet: OSPF 4.6.3 Inter-AS Routing: BGP 4.7 Broadcast and Multicast Routing 4.7.1 Broadcast Routing Algorithms 4.7.2 Multicast 4.8 Summary Homework Problems and Questions Programming Assignments Wireshark Labs: IP, ICMP Interview: Vinton G. Cerf Chapter 5 The Link Layer: Links, Access Networks, and LANs 5.1 5.2 5.3 Introduction to the Link Layer 5.1.1 The Services Provided by the Link Layer 5.1.2 Where Is the Link Layer Implemented? Error-Detection and -Correction Techniques 5.2.1 Parity Checks 5.2.2 Checksumming Methods 5.2.3 Cyclic Redundancy Check (CRC) Multiple Access Links and Protocols 5.3.1 Channel Partitioning Protocols 5.3.2 Random Access Protocols 5.3.3 Taking-Turns Protocols 5.3.4 DOCSIS: The Link-Layer Protocol for Cable Internet Access 320 322 324 326 327 331 331 332 338 353 356 362 363 366 371 379 383 384 388 390 399 400 405 412 413 429 430 431 433 434 436 437 438 440 442 443 445 448 449 459 460 Table of Contents 5.4 Switched Local Area Networks 5.4.1 Link-Layer Addressing and ARP 5.4.2 Ethernet 5.4.3 Link-Layer Switches 5.4.4 Virtual Local Area Networks (VLANs) 5.5 Link Virtualization: A Network as a Link Layer 5.5.1 Multiprotocol Label Switching (MPLS) 5.6 Data Center Networking 5.7 Retrospective: A Day in the Life of a Web Page Request 5.7.1 Getting Started: DHCP, UDP, IP, and Ethernet 5.7.2 Still Getting Started: DNS and ARP 5.7.3 Still Getting Started: Intra-Domain Routing to the DNS Server 5.7.4 Web Client-Server Interaction: TCP and HTTP 5.8 Summary Homework Problems and Questions Wireshark Labs: Ethernet and ARP, DHCP Interview: Simon S. Lam Chapter 6 Wireless and Mobile Networks 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Introduction Wireless Links and Network Characteristics 6.2.1 CDMA WiFi: 802.11 Wireless LANs 6.3.1 The 802.11 Architecture 6.3.2 The 802.11 MAC Protocol 6.3.3 The IEEE 802.11 Frame 6.3.4 Mobility in the Same IP Subnet 6.3.5 Advanced Features in 802.11 6.3.6 Personal Area Networks: Bluetooth and Zigbee Cellular Internet Access 6.4.1 An Overview of Cellular Network Architecture 6.4.2 3G Cellular Data Networks: Extending the Internet to Cellular Subscribers 6.4.3 On to 4G: LTE Mobility Management: Principles 6.5.1 Addressing 6.5.2 Routing to a Mobile Node Mobile IP Managing Mobility in Cellular Networks 6.7.1 Routing Calls to a Mobile User 6.7.2 Handoffs in GSM xxi 461 462 469 476 482 486 487 490 495 495 497 498 499 500 502 510 511 513 514 519 522 526 527 531 537 541 542 544 546 547 550 553 555 557 559 564 570 571 572 xxii Table of Contents 6.8 Wireless and Mobility: Impact on Higher-Layer Protocols 6.9 Summary Homework Problems and Questions Wireshark Lab: IEEE 802.11 (WiFi) Interview: Deborah Estrin Chapter 7 Multimedia Networking 7.1 Multimedia Networking Applications 7.1.1 Properties of Video 7.1.2 Properties of Audio 7.1.3 Types of Multimedia Network Applications 7.2 Streaming Stored Video 7.2.1 UDP Streaming 7.2.2 HTTP Streaming 7.2.3 Adaptive Streaming and DASH 7.2.4 Content Distribution Networks 7.2.5 Case Studies: Netflix, YouTube, and Kankan 7.3 Voice-over-IP 7.3.1 Limitations of the Best-Effort IP Service 7.3.2 Removing Jitter at the Receiver for Audio 7.3.3 Recovering from Packet Loss 7.3.4 Case Study: VoIP with Skype 7.4 Protocols for Real-Time Conversational Applications 7.4.1 RTP 7.4.2 SIP 7.5 Network Support for Multimedia 7.5.1 Dimensioning Best-Effort Networks 7.5.2 Providing Multiple Classes of Service 7.5.3 Diffserv 7.5.4 Per-Connection Quality-of-Service (QoS) Guarantees: Resource Reservation and Call Admission 7.6 Summary Homework Problems and Questions Programming Assignment Interview: Henning Schulzrinne Chapter 8 Security in Computer Networks 8.1 8.2 What Is Network Security? Principles of Cryptography 8.2.1 Symmetric Key Cryptography 8.2.2 Public Key Encryption 575 578 578 583 584 587 588 588 590 591 593 595 596 600 602 608 612 612 614 617 620 623 624 627 632 634 636 648 652 655 656 666 668 671 672 675 676 683 Table of Contents 8.3 Message Integrity and Digital Signatures 8.3.1 Cryptographic Hash Functions 8.3.2 Message Authentication Code 8.3.3 Digital Signatures 8.4 End-Point Authentication 8.4.1 Authentication Protocol ap1.0 8.4.2 Authentication Protocol ap2.0 8.4.3 Authentication Protocol ap3.0 8.4.4 Authentication Protocol ap3.1 8.4.5 Authentication Protocol ap4.0 8.5 Securing E-Mail 8.5.1 Secure E-Mail 8.5.2 PGP 8.6 Securing TCP Connections: SSL 8.6.1 The Big Picture 8.6.2 A More Complete Picture 8.7 Network-Layer Security: IPsec and Virtual Private Networks 8.7.1 IPsec and Virtual Private Networks (VPNs) 8.7.2 The AH and ESP Protocols 8.7.3 Security Associations 8.7.4 The IPsec Datagram 8.7.5 IKE: Key Management in IPsec 8.8 Securing Wireless LANs 8.8.1 Wired Equivalent Privacy (WEP) 8.8.2 IEEE 802.11i 8.9 Operational Security: Firewalls and Intrusion Detection Systems 8.9.1 Firewalls 8.9.2 Intrusion Detection Systems 8.10 Summary Homework Problems and Questions Wireshark Lab: SSL IPsec Lab Interview: Steven M. Bellovin Chapter 9 Network Management 9.1 9.2 9.3 What Is Network Management? The Infrastructure for Network Management The Internet-Standard Management Framework 9.3.1 Structure of Management Information: SMI 9.3.2 Management Information Base: MIB xxiii 688 689 691 693 700 700 701 702 703 703 705 706 710 711 713 716 718 718 720 720 721 725 726 726 728 731 731 739 742 744 752 752 753 755 756 760 764 766 770 xxiv Table of Contents 9.3.3 SNMP Protocol Operations and Transport Mappings 9.3.4 Security and Administration 9.4 ASN.1 9.5 Conclusion Homework Problems and Questions Interview: Jennifer Rexford References Index 772 775 778 783 783 786 789 823 COMPUTER NETWORKING SIXTH EDITION A Top-Down Approach This page intentionally left blank CHAPTER 1 Computer Networks and the Internet Today’s Internet is arguably the largest engineered system ever created by mankind, with hundreds of millions of connected computers, communication links, and switches; with billions of users who connect via laptops, tablets, and smartphones; and with an array of new Internet-connected devices such as sensors, Web cams, game consoles, picture frames, and even washing machines. Given that the Internet is so large and has so many diverse components and uses, is there any hope of understanding how it works? Are there guiding principles and structure that can provide a foundation for understanding such an amazingly large and complex system? And if so, is it possible that it actually could be both interesting and fun to learn about computer networks? Fortunately, the answers to all of these questions is a resounding YES! Indeed, it’s our aim in this book to provide you with a modern introduction to the dynamic field of computer networking, giving you the principles and practical insights you’ll need to understand not only today’s networks, but tomorrow’s as well. This first chapter presents a broad overview of computer networking and the Internet. Our goal here is to paint a broad picture and set the context for the rest of this book, to see the forest through the trees. We’ll cover a lot of ground in this introductory chapter and discuss a lot of the pieces of a computer network, without losing sight of the big picture. 1 2 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET We’ll structure our overview of computer networks in this chapter as follows. After introducing some basic terminology and concepts, we’ll first examine the basic hardware and software components that make up a network. We’ll begin at the network’s edge and look at the end systems and network applications running in the network. We’ll then explore the core of a computer network, examining the links and the switches that transport data, as well as the access networks and physical media that connect end systems to the network core. We’ll learn that the Internet is a network of networks, and we’ll learn how these networks connect with each other. After having completed this overview of the edge and core of a computer network, we’ll take the broader and more abstract view in the second half of this chapter. We’ll examine delay, loss, and throughput of data in a computer network and provide simple quantitative models for end-to-end throughput and delay: models that take into account transmission, propagation, and queuing delays. We’ll then introduce some of the key architectural principles in computer networking, namely, protocol layering and service models. We’ll also learn that computer networks are vulnerable to many different types of attacks; we’ll survey some of these attacks and consider how computer networks can be made more secure. Finally, we’ll close this chapter with a brief history of computer networking. 1.1 What Is the Internet? In this book, we’ll use the public Internet, a specific computer network, as our principal vehicle for discussing computer networks and their protocols. But what is the Internet? There are a couple of ways to answer this question. First, we can describe the nuts and bolts of the Internet, that is, the basic hardware and software components that make up the Internet. Second, we can describe the Internet in terms of a networking infrastructure that provides services to distributed applications. Let’s begin with the nuts-and-bolts description, using Figure 1.1 to illustrate our discussion. 1.1.1 A Nuts-and-Bolts Description The Internet is a computer network that interconnects hundreds of millions of computing devices throughout the world. Not too long ago, these computing devices were primarily traditional desktop PCs, Linux workstations, and so-called servers that store and transmit information such as Web pages and e-mail messages. Increasingly, however, nontraditional Internet end systems such as laptops, smartphones, tablets, TVs, gaming consoles, Web cams, automobiles, environmental sensing devices, picture frames, and home electrical and security systems are being connected to the Internet. Indeed, the term computer network is beginning to sound a bit dated, given the many nontraditional devices that are being hooked up to the Internet. In Internet jargon, all of these devices are called hosts or end systems. As of July 2011, there were 1.1 WHAT IS THE INTERNET? National or Global ISP Mobile Network Local or Regional ISP Home Network Enterprise Network Key: Host Server (= end system) Figure 1.1 Mobile Router Link-Layer switch Some pieces of the Internet Modem Base station Smartphone Cell phone tower 3 4 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET nearly 850 million end systems attached to the Internet [ISC 2012], not counting smartphones, laptops, and other devices that are only intermittently connected to the Internet. Overall, more there are an estimated 2 billion Internet users [ITU 2011]. End systems are connected together by a network of communication links and packet switches. We’ll see in Section 1.2 that there are many types of communication links, which are made up of different types of physical media, including coaxial cable, copper wire, optical fiber, and radio spectrum. Different links can transmit data at different rates, with the transmission rate of a link measured in bits/second. When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment. The resulting packages of information, known as packets in the jargon of computer networks, are then sent through the network to the destination end system, where they are reassembled into the original data. A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. Packet switches come in many shapes and flavors, but the two most prominent types in today’s Internet are routers and link-layer switches. Both types of switches forward packets toward their ultimate destinations. Link-layer switches are typically used in access networks, while routers are typically used in the network core. The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a route or path through the network. The exact amount of traffic being carried in the Internet is difficult to estimate but Cisco [Cisco VNI 2011] estimates global Internet traffic will be nearly 40 exabytes per month in 2012. Packet-switched networks (which transport packets) are in many ways similar to transportation networks of highways, roads, and intersections (which transport vehicles). Consider, for example, a factory that needs to move a large amount of cargo to some destination warehouse located thousands of kilometers away. At the factory, the cargo is segmented and loaded into a fleet of trucks. Each of the trucks then independently travels through the network of highways, roads, and intersections to the destination warehouse. At the destination warehouse, the cargo is unloaded and grouped with the rest of the cargo arriving from the same shipment. Thus, in many ways, packets are analogous to trucks, communication links are analogous to highways and roads, packet switches are analogous to intersections, and end systems are analogous to buildings. Just as a truck takes a path through the transportation network, a packet takes a path through a computer network. End systems access the Internet through Internet Service Providers (ISPs), including residential ISPs such as local cable or telephone companies; corporate ISPs; university ISPs; and ISPs that provide WiFi access in airports, hotels, coffee shops, and other public places. Each ISP is in itself a network of packet switches and communication links. ISPs provide a variety of types of network access to the end systems, including residential broadband access such as cable modem or DSL, 1.1 WHAT IS THE INTERNET? high-speed local area network access, wireless access, and 56 kbps dial-up modem access. ISPs also provide Internet access to content providers, connecting Web sites directly to the Internet. The Internet is all about connecting end systems to each other, so the ISPs that provide access to end systems must also be interconnected. These lower-tier ISPs are interconnected through national and international upper-tier ISPs such as Level 3 Communications, AT&T, Sprint, and NTT. An upper-tier ISP consists of high-speed routers interconnected with high-speed fiber-optic links. Each ISP network, whether upper-tier or lower-tier, is managed independently, runs the IP protocol (see below), and conforms to certain naming and address conventions. We’ll examine ISPs and their interconnection more closely in Section 1.3. End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received among routers and end systems. The Internet’s principal protocols are collectively known as TCP/IP. We’ll begin looking into protocols in this introductory chapter. But that’s just a start—much of this book is concerned with computer network protocols! Given the importance of protocols to the Internet, it’s important that everyone agree on what each and every protocol does, so that people can create systems and products that interoperate. This is where standards come into play. Internet standards are developed by the Internet Engineering Task Force (IETF)[IETF 2012]. The IETF standards documents are called requests for comments (RFCs). RFCs started out as general requests for comments (hence the name) to resolve network and protocol design problems that faced the precursor to the Internet [Allman 2011]. RFCs tend to be quite technical and detailed. They define protocols such as TCP, IP, HTTP (for the Web), and SMTP (for e-mail). There are currently more than 6,000 RFCs. Other bodies also specify standards for network components, most notably for network links. The IEEE 802 LAN/MAN Standards Committee [IEEE 802 2012], for example, specifies the Ethernet and wireless WiFi standards. 1.1.2 A Services Description Our discussion above has identified many of the pieces that make up the Internet. But we can also describe the Internet from an entirely different angle—namely, as an infrastructure that provides services to applications. These applications include electronic mail, Web surfing, social networks, instant messaging, Voiceover-IP (VoIP), video streaming, distributed games, peer-to-peer (P2P) file sharing, television over the Internet, remote login, and much, much more. The applications are said to be distributed applications, since they involve multiple end systems that exchange data with each other. Importantly, Internet applications 5 6 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET run on end systems—they do not run in the packet switches in the network core. Although packet switches facilitate the exchange of data among end systems, they are not concerned with the application that is the source or sink of data. Let’s explore a little more what we mean by an infrastructure that provides services to applications. To this end, suppose you have an exciting new idea for a distributed Internet application, one that may greatly benefit humanity or one that may simply make you rich and famous. How might you go about transforming this idea into an actual Internet application? Because applications run on end systems, you are going to need to write programs that run on the end systems. You might, for example, write your programs in Java, C, or Python. Now, because you are developing a distributed Internet application, the programs running on the different end systems will need to send data to each other. And here we get to a central issue—one that leads to the alternative way of describing the Internet as a platform for applications. How does one program running on one end system instruct the Internet to deliver data to another program running on another end system? End systems attached to the Internet provide an Application Programming Interface (API) that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system. This Internet API is a set of rules that the sending program must follow so that the Internet can deliver the data to the destination program. We’ll discuss the Internet API in detail in Chapter 2. For now, let’s draw upon a simple analogy, one that we will frequently use in this book. Suppose Alice wants to send a letter to Bob using the postal service. Alice, of course, can’t just write the letter (the data) and drop the letter out her window. Instead, the postal service requires that Alice put the letter in an envelope; write Bob’s full name, address, and zip code in the center of the envelope; seal the envelope; put a stamp in the upper-right-hand corner of the envelope; and finally, drop the envelope into an official postal service mailbox. Thus, the postal service has its own “postal service API,” or set of rules, that Alice must follow to have the postal service deliver her letter to Bob. In a similar manner, the Internet has an API that the program sending data must follow to have the Internet deliver the data to the program that will receive the data. The postal service, of course, provides more than one service to its customers. It provides express delivery, reception confirmation, ordinary use, and many more services. In a similar manner, the Internet provides multiple services to its applications. When you develop an Internet application, you too must choose one of the Internet’s services for your application. We’ll describe the Internet’s services in Chapter 2. We have just given two descriptions of the Internet; one in terms of its hardware and software components, the other in terms of an infrastructure for providing services to distributed applications. But perhaps you are still confused as to what the 1.1 WHAT IS THE INTERNET? Internet is. What are packet switching and TCP/IP? What are routers? What kinds of communication links are present in the Internet? What is a distributed application? How can a toaster or a weather sensor be attached to the Internet? If you feel a bit overwhelmed by all of this now, don’t worry—the purpose of this book is to introduce you to both the nuts and bolts of the Internet and the principles that govern how and why it works. We’ll explain these important terms and questions in the following sections and chapters. 1.1.3 What Is a Protocol? Now that we’ve got a bit of a feel for what the Internet is, let’s consider another important buzzword in computer networking: protocol. What is a protocol? What does a protocol do? A Human Analogy It is probably easiest to understand the notion of a computer network protocol by first considering some human analogies, since we humans execute protocols all of the time. Consider what you do when you want to ask someone for the time of day. A typical exchange is shown in Figure 1.2. Human protocol (or good manners, at least) dictates that one first offer a greeting (the first “Hi” in Figure 1.2) to initiate communication with someone else. The typical response to a “Hi” is a returned “Hi” message. Implicitly, one then takes a cordial “Hi” response as an indication that one can proceed and ask for the time of day. A different response to the initial “Hi” (such as “Don’t bother me!” or “I don’t speak English,” or some unprintable reply) might indicate an unwillingness or inability to communicate. In this case, the human protocol would be not to ask for the time of day. Sometimes one gets no response at all to a question, in which case one typically gives up asking that person for the time. Note that in our human protocol, there are specific messages we send, and specific actions we take in response to the received reply messages or other events (such as no reply within some given amount of time). Clearly, transmitted and received messages, and actions taken when these messages are sent or received or other events occur, play a central role in a human protocol. If people run different protocols (for example, if one person has manners but the other does not, or if one understands the concept of time and the other does not) the protocols do not interoperate and no useful work can be accomplished. The same is true in networking—it takes two (or more) communicating entities running the same protocol in order to accomplish a task. Let’s consider a second human analogy. Suppose you’re in a college class (a computer networking class, for example!). The teacher is droning on about protocols and you’re confused. The teacher stops to ask, “Are there any questions?” (a 7 8 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET TCP Hi conn Hi TCP GET Got the time ? 2:00 p:/ e conn /ww Time n re ctio w.a que n re wl. Time Time A human protocol and a computer network protocol message that is transmitted to, and received by, all students who are not sleeping). You raise your hand (transmitting an implicit message to the teacher). Your teacher acknowledges you with a smile, saying “Yes...” (a transmitted message encouraging you to ask your question—teachers love to be asked questions), and you then ask your question (that is, transmit your message to your teacher). Your teacher hears your question (receives your question message) and answers (transmits a reply to you). Once again, we see that the transmission and receipt of messages, and a set of conventional actions taken when these messages are sent and received, are at the heart of this question-and-answer protocol. Network Protocols A network protocol is similar to a human protocol, except that the entities exchanging messages and taking actions are hardware or software components of some device (for example, computer, smartphone, tablet, router, or other network-capable 1.2 THE NETWORK EDGE device). All activity in the Internet that involves two or more communicating remote entities is governed by a protocol. For example, hardware-implemented protocols in two physically connected computers control the flow of bits on the “wire” between the two network interface cards; congestion-control protocols in end systems control the rate at which packets are transmitted between sender and receiver; protocols in routers determine a packet’s path from source to destination. Protocols are running everywhere in the Internet, and consequently much of this book is about computer network protocols. As an example of a computer network protocol with which you are probably familiar, consider what happens when you make a request to a Web server, that is, when you type the URL of a Web page into your Web browser. The scenario is illustrated in the right half of Figure 1.2. First, your computer will send a connection request message to the Web server and wait for a reply. The Web server will eventually receive your connection request message and return a connection reply message. Knowing that it is now OK to request the Web document, your computer then sends the name of the Web page it wants to fetch from that Web server in a GET message. Finally, the Web server returns the Web page (file) to your computer. Given the human and networking examples above, the exchange of messages and the actions taken when these messages are sent and received are the key defining elements of a protocol: A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. The Internet, and computer networks in general, make extensive use of protocols. Different protocols are used to accomplish different communication tasks. As you read through this book, you will learn that some protocols are simple and straightforward, while others are complex and intellectually deep. Mastering the field of computer networking is equivalent to understanding the what, why, and how of networking protocols. 1.2 The Network Edge In the previous section we presented a high-level overview of the Internet and networking protocols. We are now going to delve a bit more deeply into the components of a computer network (and the Internet, in particular). We begin in this section at the edge of a network and look at the components with which we are most familiar—namely, the computers, smartphones and other devices that we use on a daily basis. In the next section we’ll move from the network edge to the network core and examine switching and routing in computer networks. 9 10 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET CASE HISTORY A DIZZYING ARRAY OF INTERNET END SYSTEMS Not too long ago, the end-system devices connected to the Internet were primarily traditional computers such as desktop machines and powerful servers. Beginning in the late 1990s and continuing today, a wide range of interesting devices are being connected to the Internet, leveraging their ability to send and receive digital data. Given the Internet’s ubiquity, its well-defined (standardized) protocols, and the availability of Internet-ready commodity hardware, it’s natural to use Internet technology to network these devices together and to Internet-connected servers. Many of these devices are based in the home—video game consoles (e.g., Microsoft’s Xbox), Internet-ready televisions, digital picture frames that download and display digital pictures, washing machines, refrigerators, and even a toaster that downloads meteorological information and burns an image of the day’s forecast (e.g., mixed clouds and sun) on your morning toast [BBC 2001]. IP-enabled phones with GPS capabilities put location-dependent services (maps, information about nearby services or people) at your fingertips. Networked sensors embedded into the physical environment allow monitoring of buildings, bridges, seismic activity, wildlife habitats, river estuaries, and the weather. Biomedical devices can be embedded and networked in a body-area network. With so many diverse devices being networked together, the Internet is indeed becoming an “Internet of things” [ITU 2005b]. Recall from the previous section that in computer networking jargon, the computers and other devices connected to the Internet are often referred to as end systems. They are referred to as end systems because they sit at the edge of the Internet, as shown in Figure 1.3. The Internet’s end systems include desktop computers (e.g., desktop PCs, Macs, and Linux boxes), servers (e.g., Web and e-mail servers), and mobile computers (e.g., laptops, smartphones, and tablets). Furthermore, an increasing number of non-traditional devices are being attached to the Internet as end systems (see sidebar). End systems are also referred to as hosts because they host (that is, run) application programs such as a Web browser program, a Web server program, an e-mail client program, or an e-mail server program. Throughout this book we will use the terms hosts and end systems interchangeably; that is, host = end system. Hosts are sometimes further divided into two categories: clients and servers. Informally, clients tend to be desktop and mobile PCs, smartphones, and so on, whereas servers tend to be more powerful machines that store and distribute Web pages, stream video, relay e-mail, and so on. Today, most of the servers from which we receive 1.2 THE NETWORK EDGE National or Global ISP Mobile Network Local or Regional ISP Home Network Enterprise Network Figure 1.3 End-system interaction search results, e-mail, Web pages, and videos reside in large data centers. For example, Google has 30–50 data centers, with many having more than one hundred thousand servers. 11 12 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET 1.2.1 Access Networks Having considered the applications and end systems at the “edge of the network,” let’s next consider the access network—the network that physically connects an end system to the first router (also known as the “edge router”) on a path from the end system to any other distant end system. Figure 1.4 shows several types of access National or Global ISP Mobile Network Local or Regional ISP Home Network Enterprise Network Figure 1.4 Access networks 1.2 THE NETWORK EDGE networks with thick, shaded lines, and the settings (home, enterprise, and wide-area mobile wireless) in which they are used. Home Access: DSL, Cable, FTTH, Dial-Up, and Satellite In developed countries today, more than 65 percent of the households have Internet access, with Korea, Netherlands, Finland, and Sweden leading the way with more than 80 percent of households having Internet access, almost all via a high-speed broadband connection [ITU 2011]. Finland and Spain have recently declared high-speed Internet access to be a “legal right.” Given this intense interest in home access, let’s begin our overview of access networks by considering how homes connect to the Internet. Today, the two most prevalent types of broadband residential access are digital subscriber line (DSL) and cable. A residence typically obtains DSL Internet access from the same local telephone company (telco) that provides its wired local phone access. Thus, when DSL is used, a customer’s telco is also its ISP. As shown in Figure 1.5, each customer’s DSL modem uses the existing telephone line (twistedpair copper wire, which we’ll discuss in Section 1.2.2) to exchange data with a digital subscriber line access multiplexer (DSLAM) located in the telco’s local central office (CO). The home’s DSL modem takes digital data and translates it to highfrequency tones for transmission over telephone wires to the CO; the analog signals from many such houses are translated back into digital format at the DSLAM. The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies: A high-speed downstream channel, in the 50 kHz to 1 MHz band A medium-speed upstream channel, in the 4 kHz to 50 kHz band An ordinary two-way telephone channel, in the 0 to 4 kHz band This approach makes the single DSL link appear as if there were three separate links, so that a telephone call and an Internet connection can share the DSL link at the same time. (We’ll describe this technique of frequency-division multiplexing in Home phone Internet Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz– 1MHz downstream data Splitter DSL modem Home PC Figure 1.5 DSL Internet access Central office DSLAM Telephone network 13 14 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET Section 1.3.1). On the customer side, a splitter separates the data and telephone signals arriving to the home and forwards the data signal to the DSL modem. On the telco side, in the CO, the DSLAM separates the data and phone signals and sends the data into the Internet. Hundreds or even thousands of households connect to a single DSLAM [Dischinger 2007]. The DSL standards define transmission rates of 12 Mbps downstream and 1.8 Mbps upstream [ITU 1999], and 24 Mbps downstream and 2.5 Mbps upstream [ITU 2003]. Because the downstream and upstream rates are different, the access is said to be asymmetric. The actual downstream and upstream transmission rates achieved may be less than the rates noted above, as the DSL provider may purposefully limit a residential rate when tiered service (different rates, available at different prices) are offered, or because the maximum rate can be limited by the distance between the home and the CO, the gauge of the twisted-pair line and the degree of electrical interference. Engineers have expressly designed DSL for short distances between the home and the CO; generally, if the residence is not located within 5 to 10 miles of the CO, the residence must resort to an alternative form of Internet access. While DSL makes use of the telco’s existing local telephone infrastructure, cable Internet access makes use of the cable television company’s existing cable television infrastructure. A residence obtains cable Internet access from the same company that provides its cable television. As illustrated in Figure 1.6, fiber optics connect the cable head end to neighborhood-level junctions, from which traditional coaxial cable is then used to reach individual houses and apartments. Each neighborhood junction typically supports 500 to 5,000 homes. Because both fiber and coaxial cable are employed in this system, it is often referred to as hybrid fiber coax (HFC). Coaxial cable Hundreds of homes Fiber node Fiber cable Internet CMTS Hundreds of homes Fiber node Figure 1.6 Cable head end A hybrid fiber-coaxial access network 1.2 THE NETWORK EDGE Cable internet access requires special modems, called cable modems. As with a DSL modem, the cable modem is typically an external device and connects to the home PC through an Ethernet port. (We will discuss Ethernet in great detail in Chapter 5.) At the cable head end, the cable modem termination system (CMTS) serves a similar function as the DSL network’s DSLAM—turning the analog signal sent from the cable modems in many downstream homes back into digital format. Cable modems divide the HFC network into two channels, a downstream and an upstream channel. As with DSL, access is typically asymmetric, with the downstream channel typically allocated a higher transmission rate than the upstream channel. The DOCSIS 2.0 standard defines downstream rates up to 42.8 Mbps and upstream rates of up to 30.7 Mbps. As in the case of DSL networks, the maximum achievable rate may not be realized due to lower contracted data rates or media impairments. One important characteristic of cable Internet access is that it is a shared broadcast medium. In particular, every packet sent by the head end travels downstream on every link to every home and every packet sent by a home travels on the upstream channel to the head end. For this reason, if several users are simultaneously downloading a video file on the downstream channel, the actual rate at which each user receives its video file will be significantly lower than the aggregate cable downstream rate. On the other hand, if there are only a few active users and they are all Web surfing, then each of the users may actually receive Web pages at the full cable downstream rate, because the users will rarely request a Web page at exactly the same time. Because the upstream channel is also shared, a distributed multiple access protocol is needed to coordinate transmissions and avoid collisions. (We’ll discuss this collision issue in some detail in Chapter 5.) Although DSL and cable networks currently represent more than 90 percent of residential broadband access in the United States, an up-and-coming technology that promises even higher speeds is the deployment of fiber to the home (FTTH) [FTTH Council 2011a]. As the name suggests, the FTTH concept is simple— provide an optical fiber path from the CO directly to the home. In the United States, Verizon has been particularly aggressive with FTTH with its FIOS service [Verizon FIOS 2012]. There are several competing technologies for optical distribution from the CO to the homes. The simplest optical distribution network is called direct fiber, with one fiber leaving the CO for each home. More commonly, each fiber leaving the central office is actually shared by many homes; it is not until the fiber gets relatively close to the homes that it is split into individual customer-specific fibers. There are two competing optical-distribution network architectures that perform this splitting: active optical networks (AONs) and passive optical networks (PONs). AON is essentially switched Ethernet, which is discussed in Chapter 5. Here, we briefly discuss PON, which is used in Verizon’s FIOS service. Figure 1.7 shows FTTH using the PON distribution architecture. Each home has 15 16 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET Internet Central office ONT Optical splitter ONT OLT ONT Figure 1.7 Optical fibers FTTH Internet access an optical network terminator (ONT), which is connected by dedicated optical fiber to a neighborhood splitter. The splitter combines a number of homes (typically less than 100) onto a single, shared optical fiber, which connects to an optical line terminator (OLT) in the telco’s CO. The OLT, providing conversion between optical and electrical signals, connects to the Internet via a telco router. In the home, users connect a home router (typically a wireless router) to the ONT and access the Internet via this home router. In the PON architecture, all packets sent from OLT to the splitter are replicated at the splitter (similar to a cable head end). FTTH can potentially provide Internet access rates in the gigabits per second range. However, most FTTH ISPs provide different rate offerings, with the higher rates naturally costing more money. The average downstream speed of US FTTH customers was approximately 20 Mbps in 2011 (compared with 13 Mbps for cable access networks and less than 5 Mbps for DSL) [FTTH Council 2011b]. Two other access network technologies are also used to provide Internet access to the home. In locations where DSL, cable, and FTTH are not available (e.g., in some rural settings), a satellite link can be used to connect a residence to the Internet at speeds of more than 1 Mbps; StarBand and HughesNet are two such satellite access providers. Dial-up access over traditional phone lines is based on the same model as DSL—a home modem connects over a phone line to a modem in the ISP. Compared with DSL and other broadband access networks, dial-up access is excruciatingly slow at 56 kbps. Access in the Enterprise (and the Home): Ethernet and WiFi On corporate and university campuses, and increasingly in home settings, a local area network (LAN) is used to connect an end system to the edge router. Although there are many types of LAN technologies, Ethernet is by far the most prevalent access technology in corporate, university, and home networks. As shown in Figure 1.8, Ethernet users use twisted-pair copper wire to connect to an Ethernet switch, a 1.2 THE NETWORK EDGE 100 Mbps 100 Mbps Ethernet switch Institutional router To Institution’s ISP 100 Mbps Server Figure 1.8 Ethernet Internet access technology discussed in detail in Chapter 5. The Ethernet switch, or a network of such interconnected switches, is then in turn connected into the larger Internet. With Ethernet access, users typically have 100 Mbps access to the Ethernet switch, whereas servers may have 1 Gbps or even 10 Gbps access. Increasingly, however, people are accessing the Internet wirelessly from laptops, smartphones, tablets, and other devices (see earlier sidebar on “A Dizzying Array of Devices”). In a wireless LAN setting, wireless users transmit/receive packets to/from an access point that is connected into the enterprise’s network (most likely including wired Ethernet), which in turn is connected to the wired Internet. A wireless LAN user must typically be within a few tens of meters of the access point. Wireless LAN access based on IEEE 802.11 technology, more colloquially known as WiFi, is now just about everywhere—universities, business offices, cafes, airports, homes, and even in airplanes. In many cities, one can stand on a street corner and be within range of ten or twenty base stations (for a browseable global map of 802.11 base stations that have been discovered and logged on a Web site by people who take great enjoyment in doing such things, see [wigle.net 2012]). As discussed in detail in Chapter 6, 802.11 today provides a shared transmission rate of up to 54 Mbps. Even though Ethernet and WiFi access networks were initially deployed in enterprise (corporate, university) settings, they have recently become relatively common components of home networks. Many homes combine broadband residential access (that is, cable modems or DSL) with these inexpensive wireless LAN technologies to create powerful home networks [Edwards 2011]. Figure 1.9 shows a typical home network. This home network consists of a roaming laptop as well as a wired PC; a base station (the wireless access point), which communicates with the wireless PC; a cable modem, providing broadband access to the Internet; and a router, which interconnects the base station and the stationary PC with the cable modem. This network allows household members to have broadband access to the Internet with one member roaming from the kitchen to the backyard to the bedrooms. 17 18 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET Cable head end Internet House Figure 1.9 A typical home network Wide-Area Wireless Access: 3G and LTE Increasingly, devices such as iPhones, BlackBerrys, and Android devices are being used to send email, surf the Web, Tweet, and download music while on the run. These devices employ the same wireless infrastructure used for cellular telephony to send/receive packets through a base station that is operated by the cellular network provider. Unlike WiFi, a user need only be within a few tens of kilometers (as opposed to a few tens of meters) of the base station. Telecommunications companies have made enormous investments in so-called third-generation (3G) wireless, which provides packet-switched wide-area wireless Internet access at speeds in excess of 1 Mbps. But even higher-speed wide-area access technologies—a fourth-generation (4G) of wide-area wireless networks—are already being deployed. LTE ( for “Long-Term Evolution”—a candidate for Bad Acronym of the Year Award) has its roots in 3G technology, and can potentially achieve rates in excess of 10 Mbps. LTE downstream rates of many tens of Mbps have been reported in commercial deployments. We’ll cover the basic principles of wireless networks and mobility, as well as WiFi, 3G, and LTE technologies (and more!) in Chapter 6. 1.2.2 Physical Media In the previous subsection, we gave an overview of some of the most important network access technologies in the Internet. As we described these technologies, we also indicated the physical media used. For example, we said that HFC uses a combination of fiber cable and coaxial cable. We said that DSL and Ethernet use copper wire. And we said that mobile access networks use the radio spectrum. 1.2 THE NETWORK EDGE In this subsection we provide a brief overview of these and other transmission media that are commonly used in the Internet. In order to define what is meant by a physical medium, let us reflect on the brief life of a bit. Consider a bit traveling from one end system, through a series of links and routers, to another end system. This poor bit gets kicked around and transmitted many, many times! The source end system first transmits the bit, and shortly thereafter the first router in the series receives the bit; the first router then transmits the bit, and shortly thereafter the second router receives the bit; and so on. Thus our bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs. For each transmitter-receiver pair, the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium. The physical medium can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair along the path. Examples of physical media include twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum. Physical media fall into two categories: guided media and unguided media. With guided media, the waves are guided along a solid medium, such as a fiber-optic cable, a twisted-pair copper wire, or a coaxial cable. With unguided media, the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel. But before we get into the characteristics of the various media types, let us say a few words about their costs. The actual cost of the physical link (copper wire, fiber-optic cable, and so on) is often relatively minor compared with other networking costs. In particular, the labor cost associated with the installation of the physical link can be orders of magnitude higher than the cost of the material. For this reason, many builders install twisted pair, optical fiber, and coaxial cable in every room in a building. Even if only one medium is initially used, there is a good chance that another medium could be used in the near future, and so money is saved by not having to lay additional wires in the future. Twisted-Pair Copper Wire The least expensive and most commonly used guided transmission medium is twisted-pair copper wire. For over a hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connections from the telephone handset to the local telephone switch use twisted-pair copper wire. Most of us have seen twisted pair in our homes and work environments. Twisted pair consists of two insulated copper wires, each about 1 mm thick, arranged in a regular spiral pattern. The wires are twisted together to reduce the electrical interference from similar pairs close by. Typically, a number of pairs are bundled together in a cable by wrapping the pairs in a protective shield. A wire pair constitutes a single communication link. Unshielded twisted pair (UTP) is commonly used for 19 20 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET computer networks within a building, that is, for LANs. Data rates for LANs using twisted pair today range from 10 Mbps to 10 Gbps. The data rates that can be achieved depend on the thickness of the wire and the distance between transmitter and receiver. When fiber-optic technology emerged in the 1980s, many people disparaged twisted pair because of its relatively low bit rates. Some people even felt that fiberoptic technology would completely replace twisted pair. But twisted pair did not give up so easily. Modern twisted-pair technology, such as category 6a cable, can achieve data rates of 10 Gbps for distances up to a hundred meters. In the end, twisted pair has emerged as the dominant solution for high-speed LAN networking. As discussed earlier, twisted pair is also commonly used for residential Internet access. We saw that dial-up modem technology enables access at rates of up to 56 kbps over twisted pair. We also saw that DSL (digital subscriber line) technology has enabled residential users to access the Internet at tens of Mbps over twisted pair (when users live close to the ISP’s modem). Coaxial Cable Like twisted pair, coaxial cable consists of two copper conductors, but the two conductors are concentric rather than parallel. With this construction and special insulation and shielding, coaxial cable can achieve high data transmission rates. Coaxial cable is quite common in cable television systems. As we saw earlier, cable television systems have recently been coupled with cable modems to provide residential users with Internet access at rates of tens of Mbps. In cable television and cable Internet access, the transmitter shifts the digital signal to a specific frequency band, and the resulting analog signal is sent from the transmitter to one or more receivers. Coaxial cable can be used as a guided shared medium. Specifically, a number of end systems can be connected directly to the cable, with each of the end systems receiving whatever is sent by the other end systems. Fiber Optics An optical fiber is a thin, flexible medium that conducts pulses of light, with each pulse representing a bit. A single optical fiber can support tremendous bit rates, up to tens or even hundreds of gigabits per second. They are immune to electromagnetic interference, have very low signal attenuation up to 100 kilometers, and are very hard to tap. These characteristics have made fiber optics the preferred longhaul guided transmission media, particularly for overseas links. Many of the longdistance telephone networks in the United States and elsewhere now use fiber optics exclusively. Fiber optics is also prevalent in the backbone of the Internet. However, the high cost of optical devices—such as transmitters, receivers, and switches—has hindered their deployment for short-haul transport, such as in a LAN or into the 1.2 THE NETWORK EDGE home in a residential access network. The Optical Carrier (OC) standard link speeds range from 51.8 Mbps to 39.8 Gbps; these specifications are often referred to as OCn, where the link speed equals n × 51.8 Mbps. Standards in use today include OC-1, OC-3, OC-12, OC-24, OC-48, OC-96, OC-192, OC-768. [Mukherjee 2006, Ramaswamy 2010] provide coverage of various aspects of optical networking. Terrestrial Radio Channels Radio channels carry signals in the electromagnetic spectrum. They are an attractive medium because they require no physical wire to be installed, can penetrate walls, provide connectivity to a mobile user, and can potentially carry a signal for long distances. The characteristics of a radio channel depend significantly on the propagation environment and the distance over which a signal is to be carried. Environmental considerations determine path loss and shadow fading (which decrease the signal strength as the signal travels over a distance and around/through obstructing objects), multipath fading (due to signal reflection off of interfering objects), and interference (due to other transmissions and electromagnetic signals). Terrestrial radio channels can be broadly classified into three groups: those that operate over very short distance (e.g., with one or two meters); those that operate in local areas, typically spanning from ten to a few hundred meters; and those that operate in the wide area, spanning tens of kilometers. Personal devices such as wireless headsets, keyboards, and medical devices operate over short distances; the wireless LAN technologies described in Section 1.2.1 use local-area radio channels; the cellular access technologies use wide-area radio channels. We’ll discuss radio channels in detail in Chapter 6. Satellite Radio Channels A communication satellite links two or more Earth-based microwave transmitter/ receivers, known as ground stations. The satellite receives transmissions on one frequency band, regenerates the signal using a repeater (discussed below), and transmits the signal on another frequency. Two types of satellites are used in communications: geostationary satellites and low-earth orbiting (LEO) satellites. Geostationary satellites permanently remain above the same spot on Earth. This stationary presence is achieved by placing the satellite in orbit at 36,000 kilometers above Earth’s surface. This huge distance from ground station through satellite back to ground station introduces a substantial signal propagation delay of 280 milliseconds. Nevertheless, satellite links, which can operate at speeds of hundreds of Mbps, are often used in areas without access to DSL or cable-based Internet access. LEO satellites are placed much closer to Earth and do not remain permanently above one spot on Earth. They rotate around Earth (just as the Moon does) and may communicate with each other, as well as with ground stations. To provide continuous 21 22 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET coverage to an area, many satellites need to be placed in orbit. There are currently many low-altitude communication systems in development. Lloyd’s satellite constellations Web page [Wood 2012] provides and collects information on satellite constellation systems for communications. LEO satellite technology may be used for Internet access sometime in the future. 1.3 The Network Core Having examined the Internet’s edge, let us now delve more deeply inside the network core—the mesh of packet switches and links that interconnects the Internet’s end systems. Figure 1.10 highlights the network core with thick, shaded lines. 1.3.1 Packet Switching In a network application, end systems exchange messages with each other. Messages can contain anything the application designer wants. Messages may perform a control function (for example, the “Hi” messages in our handshaking example in Figure 1.2) or can contain data, such as an email message, a JPEG image, or an MP3 audio file. To send a message from a source end system to a destination end system, the source breaks long messages into smaller chunks of data known as packets. Between source and destination, each packet travels through communication links and packet switches (for which there are two predominant types, routers and linklayer switches). Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link. So, if a source end system or a packet switch is sending a packet of L bits over a link with transmission rate R bits/sec, then the time to transmit the packet is L/R seconds. Store-and-Forward Transmission Most packet switches use store-and-forward transmission at the inputs to the links. Store-and-forward transmission means that the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link. To explore store-and-forward transmission in more detail, consider a simple network consisting of two end systems connected by a single router, as shown in Figure 1.11. A router will typically have many incident links, since its job is to switch an incoming packet onto an outgoing link; in this simple example, the router has the rather simple task of transferring a packet from one (input) link to the only other attached link. In this example, the source has three packets, each consisting of L bits, to send to the destination. At the snapshot of time shown in Figure 1.11, the source has transmitted some of packet 1, and the front of packet 1 has already arrived at the router. Because the router employs store-and-forwarding, at this instant of time, the router cannot transmit the bits it has received; instead it 1.3 National or Global ISP Mobile Network Local or Regional ISP Home Network Enterprise Network Figure 1.10 The network core THE NETWORK CORE 23 24 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET 321 R bps Front of packet 1 stored in router, awaiting remaining bits before forwarding Source Figure 1.11 Destination Store-and-forward packet switching must first buffer (i.e., “store”) the packet’s bits. Only after the router has received all of the packet’s bits can it begin to transmit (i.e., “forward”) the packet onto the outbound link. To gain some insight into store-and-forward transmission, let’s now calculate the amount of time that elapses from when the source begins to send the packet until the destination has received the entire packet. (Here we will ignore propagation delay—the time it takes for the bits to travel across the wire at near the speed of light—which will be discussed in Section 1.4.) The source begins to transmit at time 0; at time L/R seconds, the source has transmitted the entire packet, and the entire packet has been received and stored at the router (since there is no propagation delay). At time L/R seconds, since the router has just received the entire packet, it can begin to transmit the packet onto the outbound link towards the destination; at time 2L/R, the router has transmitted the entire packet, and the entire packet has been received by the destination. Thus, the total delay is 2L/R. If the switch instead forwarded bits as soon as they arrive (without first receiving the entire packet), then the total delay would be L/R since bits are not held up at the router. But, as we will discuss in Section 1.4, routers need to receive, store, and process the entire packet before forwarding. Now let’s calculate the amount of time that elapses from when the source begins to send the first packet until the destination has received all three packets. As before, at time L/R, the router begins to forward the first packet. But also at time L/R the source will begin to send the second packet, since it has just finished sending the entire first packet. Thus, at time 2L/R, the destination has received the first packet and the router has received the second packet. Similarly, at time 3L/R, the destination has received the first two packets and the router has received the third packet. Finally, at time 4L/R the destination has received all three packets! Let’s now consider the general case of sending one packet from source to destination over a path consisting of N links each of rate R (thus, there are N-1 routers between source and destination). Applying the same logic as above, we see that the end-to-end delay is: dend@to@end = N L R (1.1) You may now want to try to determine what the delay would be for P packets sent over a series of N links. 1.3 THE NETWORK CORE Queuing Delays and Packet Loss Each packet switch has multiple links attached to it. For each attached link, the packet switch has an output buffer (also called an output queue), which stores packets that the router is about to send into that link. The output buffers play a key role in packet switching. If an arriving packet needs to be transmitted onto a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer. Thus, in addition to the store-and-forward delays, packets suffer output buffer queuing delays. These delays are variable and depend on the level of congestion in the network. Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely full with other packets waiting for transmission. In this case, packet loss will occur—either the arriving packet or one of the already-queued packets will be dropped. Figure 1.12 illustrates a simple packet-switched network. As in Figure 1.11, packets are represented by three-dimensional slabs. The width of a slab represents the number of bits in the packet. In this figure, all packets have the same width and hence the same length. Suppose Hosts A and B are sending packets to Host E. Hosts A and B first send their packets along 10 Mbps Ethernet links to the first router. The router then directs these packets to the 1.5 Mbps link. If, during a short interval of time, the arrival rate of packets to the router (when converted to bits per second) exceeds 1.5 Mbps, congestion will occur at the router as packets queue in the link’s output buffer before being transmitted onto the link. For example, if Host A and B each send a burst of five packets back-to-back at the same time, then most of these packets will spend some time waiting in the queue. The situation is, in fact, entirely analogous to many common-day situations—for example, when we wait in line for a bank teller or wait in front of a tollbooth. We’ll examine this queuing delay in more detail in Section 1.4. 10 Mbps Ethernet C A 1.5 Mbps Queue of packets waiting for output link B Key: D Packets Figure 1.12 Packet switching E 25 26 CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET Forwarding Tables and Routing Protocols Earlier, we said that a router takes a packet arriving on one of its attached communication links and forwards that packet onto another one of its attached communication links. But how does the router determine which link it should forward the packet onto? Packet forwarding is actually done in different ways in different types of computer networks. Here, we briefly describe how it is done in the Internet. In the Internet, every end system has an address called an IP address. When a source end system wants to send a packet to a destination end system, the source includes the destination’s IP address in the packet’s header. As with postal addresses, this address has a hierarchical structure. When a packet arrives at a router in the network, the router examines a portion of the packet’s destination address and forwards the packet to an adjacent router. More specifically, each router has a forwarding table that maps destination addresses (or portions of the destination addresses) to that router’s outbound links. When a packet arrives at a router, the router examines the address and searches its forwarding table, using this destination address, to find the appropriate outbound link. The router then directs the packet to this outbound link. The end-to-end routing process is analogous to a car driver who does not use maps but instead prefers to ask for directions. For example, suppose Joe is driving from Philadelphia to 156 Lakeside Drive in Orlando, Florida. Joe first drives to his neighborhood gas station and asks how to get to 156 Lakeside Drive in Orlando, Florida. The gas station attendant extracts the Florida portion of the address and tells Joe that he needs to get onto the interstate highway I-95 South, which has an entrance just next to the gas station. He also tells Joe that once he enters Florida, he should ask someone else there. Joe then takes I-95 South until he gets to Jacksonville, Florida, at which point he asks another gas station attendant for directions. The attendant extracts the Orlando portion of the address and tells Joe that he should continue on I-95 to Daytona Beach and then ask someone else. In Daytona Beach, another gas station attendant also extracts the Orlando portion of the address and tells Joe that he should take I-4 directly to Orlando. Joe takes I-4 and gets off at the Orlando exit. Joe goes to another gas station attendant, and this time the attendant extracts the Lakeside Drive portion of the address and tells Joe the road he must follow to get to Lakeside Drive. Once Joe reaches Lakeside Drive, he asks a kid on a bicycle how to get to his destination. The kid extracts the 156 portion of the address and points to the house. Joe finally reaches his ultimate destination. In the above analogy, the gas station attendants and kids on bicycles are analogous to routers. We just learned that a router uses a packet’s destination address to index a forwarding table and determine the appropriate outbound link. But this statement begs yet another question: How do forwarding tables get set? Are they configured by 1.3 THE NETWORK CORE hand in each and every router, or does the Internet use a more automated procedure? This issue will be studied in depth in Chapter 4. But to whet your appetite here, we’ll note now th

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